In our International Patent Application No. PCT/AU2010/001052 (published as WO 2011/020143), the entirety of which is herein incorporated, there are disclosed flow meter assemblies and methods of flow measurements.
In traditional flow measurement technologies (such as electromagnetic flow meters) flow is determined by multiplying the known cross-sectional area of a pipe or channel by the average velocity passing through this known cross section. Typically, there is one flow velocity sensor, and the average velocity is determined using this sensor. Flow is derived by multiplying the total cross sectional area of the said pipe or channel by this average velocity. The problem with this measurement technology is that the use of the average velocity multiplied by the total cross sectional area allows significant errors to occur. Unfortunately, silt may build up in the pipe or channel, reducing the cross-sectional area of the pipe or channel. Because the area through which fluid flows in a silted pipe or channel is reduced relative to a clean pipe or channel, the area assumed in the flow measurement calculation is greater than the true cross-sectional area through which the fluid flows. The flow continues to be calculated by multiplying the average velocity by the assumed cross-sectional area of the conduit. This will result in significant errors in determining the flow rate.
The above problems were lessened using the systems disclosed in our International Patent Application No. PCT/AU2010/001052. This system provided a flow meter that uses the ‘time of flight’ acoustic or ‘transit time’ method to measure multiple velocities at multiple slices through the cross-sectional area of the flow meter. The system provided a multi-path analysis of velocity across a pipe or channel at a number of horizontally disposed layers. The method of computing flow is to first compute the velocity within each discrete horizontal layer. The velocity within each layer is then multiplied by the width and the height of that layer to determine the flow passing through that layer. The flows passing through each layer are then summed to determine the total flow passing through the cross-section of the meter. The flow through the conduit is therefore the sum of each discrete flow layer. Such a calculation, using multiple sensors, provided an accurate determination of flow.
When silt accumulates, the cross sectional area of the conduit changes. At the same time, the actual velocity profile within this cross-section changes. Because the flow velocity at the silt-water interface is zero, the bottom path velocity decreases. To maintain the same flow rate through the pipe or channel, the remaining path velocities must increase slightly. Because multiple velocity measurements are made at known elevations within the meter cross section, the actual velocity profile is used to calculate the flow rate. The error in the calculated flow will be reduced compared with the traditional flow measurement technology previously described which uses a single velocity measurement to compute flow.